Significance These studies should provide insight into the use of recombinant Listeriamonocytogenes for generating anti-viral mucosal immunity in primates. Because live-attenuated bacterial vaccines are currently used in people, this work may lead directly to a vaccine that can be used to prevent the sexual transmission of HIV. Objectives This project will use the SIV/macaque model to test the efficacy of immunization with recombinant Listeria monocytogenes, an attenuated intracellular bacterium. Initial studies will characterize the pathogenicity in rhesus monkeys of attenuated listeria vectors, and immune responses of rhesus macaques to the proteins of SIV expressed by these vectors. Anti-SIV cytotoxic T lymphocyte responses in peripheral blood, lymphoid tissues and the genital mucosa will be quantified. And systemic and secretory (including vaginal) immune responses to the immunizations will be characterized using isotype and SIV-specific assays. The immune response between each boost will be measured in order to assess which components of the immunization protocol are essential to obtain a protective response. If a strong local immune response is produced by the immunization strategy, the animals will be vaginally challenged with virulent SIV. Results Initial immunization of two juvenile monkeys with listeria expressing the SIV gag gene was found to be nontoxic, with doses ranging from 105 to 108 colony forming units given intramuscularly. These monkeys became infected with SIV after oral challenge, but the viral loads were relatively low in blood and lymph node. Four more monkeys have been immunized with control or SIV-expressing listeria based on the initial experiment, and they will be challenged orally with SIV 3 weeks after the third immunization. Future Directions Of particular interest is whether these intracellular bacteria are capable of inducing cellular immune responses in the genital tract. KEYWORDS SIV, vaccine, cell - mediated immunity;